1. Field of the Invention
This invention relates to a miniaturized atomic frequency standard of the type in which a microwave oscillator is tuned to the hyperfine frequency of a vapor pumped by a laser diode, and more particularly to control of the oscillator, laser diode and associated components using a digital processor.
2. Background of Information
A known type of atomic clock tunes a microwave oscillator to the hyperfine wavelength of a vapor such as cesium or rubidium. A light beam passed through the vapor pumps the atoms from the ground state to a higher state from which they fall to a state which is at a hyperfine wavelength above the ground state. Absorption of the light in pumping the vapor atoms to the higher state is detected by a photodetector. When the ground state becomes depleted the light passing through the vapor to the detector increases. However, with the microwave signal tuned to the hyperfine wavelength, the ground state is repopulated so that light is continuously absorbed in pumping the vapor atoms. Thus, the response of the detector to the light signal exhibits a dip at the exact wavelength at which the vapor atoms are pumped to the higher state. The microwave frequency must also be at the precise hyperfine frequency to produce the maximum absorption of the light, and therefore, the minimum photodetector signal.
With a stable light source, the microwave oscillator is precisely tuned to the hyperfine frequency using the photodetector output circuit in a feedback loop. In order to lock onto the dip and therefore the hyperfine frequency, the microwave frequency is dithered, that is, it is modulated between two frequencies. The feedback circuit drives the microwave frequency to center the dithered response signal of the photodetector on the precise hyperfine frequency.
The laser beam is also stabilized by a feedback circuit which dithers the laser wavelength to lock the laser on the precise wavelength which pumps the vapor atoms to the excited state. The analog feedback circuits controlling the microwave frequency and laser wavelength use frequency multiplexing to decouple modulation of the two signals. This requires separate phase detectors for each of the feedback loops.
There is a trend to reduce the size and power requirements of atomic frequency standards which would significantly expand their usefulness. U.S. Pat. No. 5,192,921 discloses a miniature cell type atomic frequency standard in which the gas cell is no more than about 6 mm in diameter and 18 mm long. When cesium gas is used, known analog circuits as described above can be used with this gas cell to produce a frequency standard with a total volume of less than about 25 cm.sup.3. However, the separate phase detectors needed to extract the respective feedback signals for the microwave and laser control loops restrict further miniaturization of these analog circuits.
There is a need therefore for an improved atomic frequency standard with a smaller total size, including control circuits, than is now available.
There is a related need for such a reduced size atomic frequency standard which is stable and reliable, and can be produced for a reasonable cost.
There is a further need for such an atomic frequency standard which has reduced power requirements both as to amount of power drawn and quality of power needed.